Study On The Structure Characteristics Of The Vermiculite Supplied By The Weili Mine Of Xinjiang And Mechanisms For The Metal Ions And The Phosphate Adsorption On The Vermiculite

Natural self-purification, a potential mechanism in nature whereby mankind and the earth are interrelated with each other, has been playing an increasingly important role in the field of harnessing contamination and remedying environment. Metal pollutants treatment by natural clay minerals is based on the law of nature and reflects natural self-purification function in the inorganic world. Vermiculite is one of the natural clay minerals, which is relatively cheap and easily available in China. Although previous researches have highlighted the utility of this very low cost and environmentally friendly vermiculite in the removal of heavy metals and phosphates from wastewater, little attention has been paid to the mechanisms of kinetics and the thermodynamics for the metal adsorption on natural vermiculite and for the phosphate adsorption on the modified vermiculite.Therefore, the objective of this paper was to investigate the structure characteristics of the vermiculite supplied by the Weili mine of Xinjiang by using kinds of analysis methods based on the knowledge of environmental chemistry, analysis chemistry and mineralogy. Moreover, the kinetics and thermodynamics mechanisms of Ag⁺, Pb²⁺, Cd²⁺ and Ni²⁺ adsorption on natural vermiculite were studied. Furthermore, two new kinds of adsorption materials of Al-pillared vermiculite and CaCl₂ modified vermiculite were prepared, and phosphates sorption experiments were carried out for the two modified vermiculites to investigate sorption behaviors and mechanisms of phosphates.The structure of an adsorbent is the key to decide itsadsorption ability. By using SEM, FT-IR, TG and XRD methods, the structure characteristics of the vermiculite were investigated, which indicated that the vermiculite is not pure vermiculite but a mixture of hydrobiotite and vermiculite. In addition, a new method for computing the proportion of phlogopite and vermiculite crystal layer of phlogopite-vermiculite interstratified mineral from Weili mine of Xinjiang was established based on the analysis of XRD, which is better than the traditionai method of individual crystal layer chemical formula. Furthermore, the tridimentional models of the vermiculite layer and the hydrobiotite crystal layer of Weili vermiculite of Xinjiang were established firstly by the MS modeling software.Based on the systemic investigation of the structure characteristics of the vermiculite, the kinetics and thermodynamics mechanisms of Ag⁺, Pb²⁺, Cd²⁺, Ni²⁺ adsorption on natural vermiculite were studied. The kinetics of Pb²⁺ adsorption on vermiculite can be best described by pseudo-first order kinetics model and pseudo-second order kinetics model. The kinetics of Ag⁺, Cd²⁺ and Ni²⁺ adsorption on vermiculite Can be best described by pseudo-second order kinetics model. The adsorption capacities calculated by the model were consistent with those actual measurements, which the correlation coefficient (R²) was more than 0.99 at room temperature. The adsorption rates for metal ions were in the order of Ag⁺＞Pb²⁺＞＞Cd²⁺＞Ni²⁺. The adsorption isotherms for Ag⁺, Pb²⁺, Cd²⁺ and Ni²⁺ adsorption on vermiculite followed both Langmuir and Freundlich isotherm model, which the correlation coefficients (R²) were more than 0.9 at room temperature. The adsorption capacities for metal ions were in the order of Pb²⁺＞Cd²⁺＞Ag⁺＞Ni²⁺. Furthermore, the effects of temperature on the Pb²⁺, Ag⁺ adsorption on vermiculite were studied. Increase of temperature from 10℃to 80℃increased the sorption of Pb²⁺ and Ag⁺, indicating the process to be endothermic. Thermodynamic parameters such as change in enthalpy (â–³H), change in entropy (â–³S), and change in free energy (â–³G) at different temperature were firstly evaluated by applying the Van't Hoff equations. In addition, the XRD patterns for vermiculite adsorbed different metal ions, such as Pb²⁺, Ag⁺, Cd²⁺, Ni²⁺ indicated that the d₀₀₁ space was different from one metal to another, which were related to many factors, such as electron conformation, ion charges and ionic radius. The TG and XPS for vermiculite adsorbed Pb²⁺ indicated that there were more Pb(â…¡) forms adsorption on vermiculite, such as Pb-O-Si, Pb-OH, Pb(OH)₂ and so on.Moreover, the competitive adsorption capacity of metal ions Ag⁺, Pb²⁺, Cd²⁺, Ni²⁺ in a binary, ternary and quadruple systems on vermiculite followed Pb²⁺＞Cd²⁺＞Ni²⁺＞Ag⁺. Such behaviors were determined by the ion charge, the hydrated ionic radius and the hydration energies of metal species. The adsorption equilibrium of Pb²⁺ in a binary, ternary and quadruple system on vermiculite can be described by the Langmuir isotherm model. The adsorption rate among the four metal ions adsorbed onto vermiculite followed Ag⁺＞pb²⁺＞Ni²⁺＞Cd²⁺. The pseudo-second order kinetics model best described the kinetics of four metals. The adsorption capacities calculated by the model were consistent with those actual measurements, which the correlation coefficient (R²) were more than 0.98.In order to improve the adsorption capacity of Ag⁺ on vermiculite, the natural vermiculite was modified by heat-treatment and acid-treatment. After heated at different temperature, with temperature increasing from 100℃to 600℃, the cation exchanged capacity (CEC) of the vermiculite decreased to almost zero, and the peak of d₀₀₁ decreased in intensity. The adsorption capacity of Ag⁺ for the vermiculite heated at 100℃was higher than that for the untreated vermiculite, while the adsorption capacities of Ag⁺ for the vermiculite heated at 200℃, 300℃, 600℃respectively were lower than that for the untreated vermiculite. The Ag⁺ adsorption on the vermiculite heated at 100℃obeyed both Langmuir and Freundlich model, and the correlation coefficients (R²) were 0.999 and 0.975 at room temperature, respectively.In addition, after leached with different concentration of HCl, with HCl concentration increasing from 0.1mol/L to 2.0mol/L, the peak of d₀₀₁ for vermiculite decreased to dismiss in intensity, and the cation exchange capacity (CEC) decreased to almost zero. The adsorption capacity of Ag⁺ for the vermiculite leached with 0.1 mol/L HCl was 10% higher than that for the untreated vermiculite, while the adsorption capacities of Ag⁺ for the vermiculites leached with 0.5 mol/L, 1.0 mol/L, 2.0 mol/L HCl respectively were lower than that for the untreated vermiculite. The Ag⁺ adsorption on the vermiculite leached with 0.1 mol/L HCl obeyed both Langmuir and Freundlich model, and the correlation coefficients (R²) were 0.961 at room temperature, respectively.Because of the poor adsorption ability of phosphates on natural vermiculite, two new kinds of adsorption materials of Al-pillared vermiculite and CaCl₂ modified vermiculite were prepared. A new method for preparation of Al-pillared vermiculite using the natural vermiculite supplied from Weili mine of Xinjiang has been established. The natural vermiculite was submitted to a nitric acid treatment followed by calcination, oxalic acid treatment, and sodium form by ion exchange, which results in an increase of the framework Si/Al ratio, thus, the reduction of the layer change density, and the conversion of the vermiculite into a form of sodium-vermiculite. Then the 18A Al-pillared vermiculite was obtained by contact with the aluminum pillaring solution. Furthermore, phosphate sorption experiments were carried out for Al-pillared vermiculite and untreated vermiculite under different pH condition to investigate sorption behaviors and mechanisms of phosphates. The phosphate sorption maxima for the Al-pillared vermiculite were 11.51% at pH 3, which were higher by about 3 orders than those for natural vermiculite.A new method for CaCl₂ modified vermiculite adsorption phosphate has been established. The adsorption ratio of phosphate with P concentration of 20 mg/L on the 0.1g vermiculite modified by CaCl₂ was 98.5 %. The phosphate sorption to the untreated and CaCl₂ modified vermiculite strongly depended on pH. For the untreated vermiculite, the phosphate adsorption was lower under pH less 10.6. From pH 10.6 to pH 13, the phosphate adsorption increased largely. From pH 12 to pH 13, the phosphate adsorption reached a plateau value and remained constant, ranging from 66 % to 66.5 %. For the CaCl₂ modified vermiculite, the phosphate adsorption was higher than that the untreated vermiculite with different pH. At pH less 4, the phosphate sorption was 4 %. At pH 8.0, the phosphate sorption increased to 5%. From pH 9.5 to pH 13, the phosphate sorption increased sharply, reached 99% with pH-12. The kinetics of phosphate adsorption on CaCl₂ treated vermiculite and untreated vermiculite can be best described by pseudo-first order kinetics model and pseudo-second order kinetics model. The adsorption capacities calculated by the model were consistent with those actual measurements, which the correlation coefficient (R²) were more than 0.98. The overall rate constants for phosphate adsorbed on modified vermiculite and untreated vermiculite were 0.1289 min^-1 and 0.07765 min ^-1 respectively by pseudo-first order kinetics model. The rate constants calculated by pseudo-second order kinetics model for phosphate adsorption on modified vermiculite and untreated vermiculite were 0.2809 g·mg^-1·min^-1 and 0.2273 g·mg^-1·min^-1 respectively. The adsorption isotherms for CaCl₂ modified vermiculite and untreated vermiculite followed both Langmuir and Freundlich isotherm model, which the correlation coefficients (R²) were more than 0.9 at room temperature. Increase of temperature from 10℃to 60℃increased the sorption capacity of P from 11.29 mg/g to 12.79 mg/g by CaCl₂ modified vermiculite and from 3.856 mg/g to 5.000 mg/g by untreated vermiculite, indicating that CaCl₂ modified vermiculite was better than untreated vermiculite to remove phosphate. For the CaCl₂ modified vermiculite and untreated vermiculite, theâ–³H were 4.072 kJ/mol, 128.7 kJ/mol respectively, which indicated that rise in temperature favored the adsorption and the adsorption process was an endothermic reaction in nature. Theâ–³S were 25.71 J/mol K, 461.7 J/mol K respectively, and theâ–³G were negative at different temperature. It indicated that the adsorption was spontaneous in the nature and the spontaneous increased with increasing temperature. This observation can be explained by the fact that for the CaCl₂ modified vermiculite and untreated vermiculite, there are more than one mechanism for phosphate adsorption. Along with the usual physisorption, the chemisorption of phosphate at the active sites in the vermiculite surface, such as P precipitation induced by Ca²⁺ also took place. Increasing temperature not only increased the active surface centers available for adsorption, but also decreased the activation energy for the adsorption. All of these are favor to chemisorption.